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Is entanglement still considered spooky ?

  1. Jan 31, 2012 #1
    Is entanglement still considered "spooky"?

    Even now in 2012, you often hear the phrase "spooky action at a distance" regarding entanglement. Is this just a popular-science cliché, or do some physicists really consider EPR effects (e.g. Alice sees spin-up, therefore Bob must see spin-down) somehow mysterious?

    This may be my naïve view, but I see this phenomenon as a conservation issue, a generalization of Newton's 3rd law, where the "action" and "reaction" in this case are spacelike separated. What subtlety am I missing by viewing a Bell-test result as analogous to a classical cannon shot over here and a recoil over there?

    To put it another way: Even if no hidden variables are involved, and regardless of distance, is anyone surprised that the universe constrains Alice and Bob's experiments to abide by ordinary laws of conservation?
     
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  3. Jan 31, 2012 #2

    DaveC426913

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    Re: Is entanglement still considered "spooky"?

    Surprised, no. Mystified, yes.
    The mystery is: how??
     
  4. Jan 31, 2012 #3
    Re: Is entanglement still considered "spooky"?

    Exactly, we may know what happens but why is it happening? Spooky indeed..
     
  5. Jan 31, 2012 #4
    Re: Is entanglement still considered "spooky"?

    One particularly aspect I'm not clear on: after making a measurement and checking the other particle, they have then remeasured the first again, observed it has changed and then that the second has also changed?
     
  6. Jan 31, 2012 #5
    Re: Is entanglement still considered "spooky"?

    salvestrom, for an excellent explanation of what entanglement is all about and what makes it so weird, see this article by Nick Herbert:http://quantumtantra.com/bell2.html

    Most people, when they first hear about entanglement, make the same assumption you did, that it's just conservation laws, e.g. one particle has one spin, then conservation of angular momentum would lead to the other particle having the other path. But if this was really what entanglement was about, there would be nothing "quantum" about it. You could replicate this trivial kind of "entanglement" classically by writing your name on a piece of paper, cutting the paper in half and randomly putting the two halves in separate envelopes that are sent to distant locations. Then if one of the envelopes is found to contain the left half of the paper, the other one is guaranteed to have the right half: you can call it "conservation of paper".

    No, this is not at all the nature of quantum entanglement, which is a much more subtle and mysterious phenomenon. To find out the difficulties encountered if you viewed entanglement as similar to my envelopes example, see the link I referred to above.
     
  7. Feb 1, 2012 #6
    Re: Is entanglement still considered "spooky"?

    Just imagine if we had quantum entaglement technology. You could literally make a cell phone call from anywhere in the universe!
     
  8. Feb 1, 2012 #7

    Ryan_m_b

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    Re: Is entanglement still considered "spooky"?

    No you couldn't
     
  9. Feb 1, 2012 #8
    No, the same theory of quantum mechanics that tells us about the wonders of entanglement also tell us that it cannot be used to transmit information faster than possible by other means (i.e. the speed of light).
     
  10. Feb 1, 2012 #9
    Re: Is entanglement still considered "spooky"?

    Thanks for the responses. I've always found the folks here to be very helpful.

    I guess I just don't take the idea of spatial separation very seriously. The holographic picture appeals to me. On FQXi recently there was a neat argument, based on quantum cryptography, for why the idea of spatial positioning is deeply problematic:
    http://www.fqxi.org/community/forum/topic/1238
     
  11. Feb 2, 2012 #10
    Re: Is entanglement still considered "spooky"?

    The why is... "Something must happen" otherwise there is an abscence of interaction & and that something is the why, built out of a unique set of varables.
     
  12. Feb 2, 2012 #11
    Re: Is entanglement still considered "spooky"?

    What does anyone else make of the above and the included link?

    Personally, I find nothing mysterious about the described experiment, which is more like the envelope anology. Synchronised photons doesn't seem like much of an issue. My description which I thought was the actual implication of quantum entanglement would be genuinely weird, if true.

    Nick Herbert's wikipage and own website are not encouraging. In the linked article he declares the non-linear jump from 25% to 75% inaccuracy as proof of non-localality without explaining why. Seems odd to conclude anything at all given there is a diagram showing how the relationship varies.

    Does someone have a link to another rundown that doesn't, at the bottom, have a link to another page about placing limits on psychic powers?
     
  13. Feb 2, 2012 #12
    Re: Is entanglement still considered "spooky"?

    The distinction between the entanglement phenomenon and a classical situation (such as the envelope example) is subtle. What many pop-science writers don't go into is that a particle can be measured in any manner of ways, and its entangled twin will always turn up opposite. If you measure spin up along 0°, the twin will definitely be spin-down along 0°, or along 90° a 50-50 chance of being spin-up or spin-down. If you then measure another particle along 45°, its twin will definitely be opposite along 45° or 50-50 along 135°. And so on. So, in order for this to be happening in a classical manner (like the envelope), it seems as if each particle would have to carry an inordinate amount of information to account for any possible measurement result -- or, one particle would need to "signal" the other regarding the measurement that was just performed. Since neither explanation is satisfying (especially since Aspect and other tests show that such signaling would be superluminal), the phenomenon is deemed weird.

    My original question was whether the physics community is still really "spooked," as various interpretations and the holographic principle claim to account for these things. For example, the fact that any "spooky" test results can only be verified through ordinary, local communication channels (as in relational QM).

    A fairly extensive and readable run-down of entanglement, written by Jeffrey Bub, can be found here:
    http://plato.stanford.edu/entries/qt-entangle/
     
    Last edited: Feb 2, 2012
  14. Feb 2, 2012 #13
    Re: Is entanglement still considered "spooky"?

    No one can ever answer the why of something only the how or how again.

    I think something like that should be in the fundaments of physics wasking why never gets you anywhere, this is familiar to when we were childern. Every thime you get an answer and you can ask why again it never stops and doesn't lead to anymore real understanding
     
  15. Feb 2, 2012 #14
    Re: Is entanglement still considered "spooky"?

    Turning up opposite doesn't seem particularly wierd. They're entangled, after all. What I'm trying to ascertain from all this is if it is being said, or has even been experimentally demonstrated, that you can remeasure the first particle, get a different result and then find that the second particle has also appropriately altered. That would be "wierd".
     
  16. Feb 2, 2012 #15
    Re: Is entanglement still considered "spooky"?

    You can't remeasure the particle and get a different result on the same observable -- the wavefunction is already "collapsed."

    I agree with you that entanglement isn't particularly weird, but you kind of have to throw out the idea that spatial separation is a fundamental feature of the world. Not everyone is ready to do that.
     
  17. Feb 2, 2012 #16
    Re: Is entanglement still considered "spooky"?

    Hmm. See, I'd start by arguing from the corner that the wavefunction was collapsed at the point of entanglement, hence the outcome of any later measurement is pre-set. Unfortunately, I'm pretty sure that would be impossible to prove and ties in with other debates about the reality of the wave-function, its collapse, etc...
     
  18. Feb 2, 2012 #17
    Re: Is entanglement still considered "spooky"?

    Well if the wavefunction has collapsed at the point of entanglement, then it would have to collapse with an infinite number of observable values, which kind of wipes out the idea of collapse in the first place (a single, definite result for a particular observable).

    This is because prior to measurement, the entangled particle has the potential to yield a measurement result along any axis, and its twin, measured along the same axis, simultaneously has the potential to be measured opposite. The choice of axis is totally arbitrary. This can't be explained with any classical-type picture.
     
  19. Feb 2, 2012 #18

    zonde

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    Re: Is entanglement still considered "spooky"?

    The part between "Here's how it works." and "Therefore the locality assumption is false. Reality must be non-local." is good. It really explains why people think that entanglement is weird. This is as simple as possible so if you do not get it try to ask more specific questions about this explanation.

    But let me add that the story does not end here. That's because there are differences between theory and experiment that can make a difference.

    Realistic treatment of that setup says you have 50% mismatch maximum in step 4 given that observations at step 1,2,3 are correct. Experiments suggest 75% mismatch in step 4 (and of course steps 1,2,3 as described).
     
  20. Feb 3, 2012 #19
    But the whole point of Herbert's article was to show how you run into difficulties if you assume that entanglement has an envelope-type explanation. Let me summarize the proof for you.

    If you send the two photons through detectors oriented at 0 degrees, you get 100% correlation, which might suggest to you that the polarizations of the two photons was set initially before they went apart, akin to the envelopes. If you turn one of the detectors by 30 degrees, you get a 25% error rate. No problem, you might think: it may just be because the fact that the detector has been turned leads to the polarization of the photon changing 1 out of every 4 times. So now turn both detectors by 30 degrees in opposite directions. Now they should each have a 25% error rate, so at most they should have an error rate of 50% (of course it should be even less than that, because if both of them have an error at the same time the two errors cancel each other out). Yet we find that the error rate is 75%, contradicting the conclusion we reached assuming an envelope-type explanation.

    And yes, the rest of Herbert's website is rather weird, but this article is perfectly good. He has published this simple proof of Bell's theorem in journals, and in fact this particular numerical example involving 30 and 60 degrees was the one Bell himself used to use to demonstrate his theorem to popular audiences.
     
    Last edited: Feb 3, 2012
  21. Feb 3, 2012 #20
    Re: Is entanglement still considered "spooky"?

    I agree sometimes it can be the bain of things. MY G.F did it to me last night like a child over and over again, Why, Why, Why, & every time I gave an answer. Then it got to a point where I could not answer why! At that point I guess, that is the goal post that research is fumbling with. We can answer why to a certain point, then it gets tricky. Why did I stand up? Easy...Why did the weather change...easy..why does light travel? Hard.
     
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